1. Field: This invention relates to a composition and method of manufacturing a sintered silicon carbide ceramic body. More particularly, the invention relates to the addition of various mineral oxides to promote the formation of a liquid phase during the densification of silicon carbide.
2. Statement of the Art: Silicon based ceramics are leading candidates for applications in high temperature environments, such as energy conversion devices, due to their high strength at temperatures on the order of 1500.degree. centigrade (.degree.C). Silicon carbide (SiC) and silicon nitride (Si.sub.3 N.sub.4) also find use in application over a wide temperature range, where either wear resistance or corrosion resistance is required.
Sintering in the absence of applied pressure ("pressureless sintering") of SiC has been accomplished using sintering aids. Sintering aids include boron (B), carbon (C) or boron carbide (B.sub.4 C) and/or aluminum (or alumina). Sintering aids have been used to obtain nearly single phase SiC with densities greater than 97% of theoretical. (Theoretical density, as is well known in the art, is the density of a fully dense ceramic body.) Very active powders having high surface areas (mean particle size less than 0.5 micrometers) have been heretofore needed to provide the driving force for pressureless sintering.
Sintering of SiC through solid state diffusion typically takes place at temperatures of at least 2050.degree. C. and at times exceeding 30 minutes. Very little densification of SiC particles having sizes greater than one micrometer occurs by the prior art methods of pressureless sintering. Since pressureless sintering allows for the fabrication of complex shapes economically, it would be an improvement in the art if SiC particles in the 1 micrometer range, which are easier to manufacture and handle than 0.5 micrometer particles, could be densified without applied pressure.
Silicon nitride is typically densified, aided by a liquid phase, at temperatures ranging between 1500.degree. C. to 1850.degree. C. for times of at least 30 minutes. The presence of a liquid phase is critical to the process since it allows alpha silicon nitride to be converted into beta silicon nitride. The alpha silicon nitride goes into solution and precipitates out as beta silicon nitride. This treatment leads to an acicular microstructure with increased, fracture toughness. Typical sintering aids used to densify Si.sub.3 N.sub.4 include MgO, Y.sub.2 O.sub.3 --Al.sub.2 O.sub.3, Y.sub.2 O.sub.3 --ZrO.sub.2, CeO.sub.2, and CaO. These oxides and others react with silica present on the surface of the silicon nitride to form a liquid phase at temperatures below 1850.degree. C. Decomposition of Si.sub.3 N.sub.4 is excessive at temperatures greater than 1850.degree. C. Although Si.sub.3 N.sub.4 has greater strength and toughness than SiC and is therefore more resistant to catastrophic failure, SiC has higher hardness and is therefore preferred in wear applications. Also, SiC has a higher resistance to creep resistance which may be beneficial in heat engine applications.
Silicon carbide has been sintered to high density and strength using rare earth oxides as additives. U.S. Pat. Nos. 4,502,983 to Omori, et al.; 4,564,490 to Omori et al.; and 4,569,921 to Omori et al. disclose the use of rare earth oxides to promote solid state sintering. These inventions require the use of SiC of submicron size and typically result in surfaces having higher concentrations of rare earth oxides. Virkar, et al. (U.S. patent application Ser. No. 778,251) discloses a method for densifying a mixture of SiC and SiCAlON (a solid solution of SiC, Al.sub.2 OC and AlN) using a liquid phase provided by the carbothermal reduction of alumina (Al.sub.2 O.sub.3). These above techniques do not result in SiC having high hardness, i.e., hardness exceeding 23 gigapascals (GPa) or improved wear resistance.
In this disclosure, unless otherwise indicated, all quantities, proportions, and ratios are stated on a weight basis. All references to "mesh" are to U.S. mesh.